1. Field of the Invention
The invention relates generally to image decoding, and more particularly to an image decoding apparatus and method for performing high-speed processing for low display resolutions.
2. Description of the Related Art
The Joint Photographic Expert Group (JPEG) format is a commonly used image compression format. The JPEG format benefits multimedia communications, as images with large number of pixels can be compressed using the JPEG format to be compatible with digital cameras, mobile phones or computers. Although the JPEG format advantageously reduces storage facility requirements for images, enormous amounts of computations and time are required for encoding and decoding, especially for high throughput demands.
To decompress a JPEG compressed bit stream, the ordinary decoding process includes header parsing, Huffman decoding, de-quantization, Inverse Discrete Cosine Transform (IDCT), downscaling, and color space converting. In general, the resolution of images captured and pre-stored by digital cameras is significantly larger than the resolution for display devices, such as televisions or printers. While being converted or decoded to a lower resolution from a higher resolution, unnecessary detailed information is dropped from the original images. For example, U.S. Pat. Nos. H1684, 6,067,384 and 7,050,656 disclose some techniques for improving decoding efficiency during the IDCT and the downscaling process.
Additionally, various techniques, e.g., U.S. Pat. Nos. RE039,925, 6,798,365 and 5,825,312, have been proposed to improve the decoding efficiency during the Huffman decoding process. However, all encoded data for a unit block of an original image, such as DCT coefficients for an 8 by 8 block, still need to be fully derived or decoded during the Huffman decoding process.
FIG. 1 is a flowchart of a conventional Huffman decoding process 10 illustrating operations of decoding an 8 by 8 block. Generally, each 8 by 8 block of an original image is transformed into sixty-four frequency coefficients, called DCT coefficients, consisting of one DC coefficient and subsequently sixty-three AC coefficients. When a specific unit of a bit stream corresponding to the block is acquired, a DC decoding process is first performed on the specific unit to obtain a DC coefficient for the block (step S102). Further, an AC decoding process is implemented to decode the remaining specific unit and generate AC coefficients for the block (step S104). Meanwhile, a determination process is provided for determining whether all AC coefficients for the block have been decoded (step S106). That is, the conventional Huffman decoding process 10 is only completely finished when sixty-three AC coefficients for the block are generated. Since the operation of the AC decoding process is redundant and time-consuming, it becomes impractical when used for low display resolutions, wherein a limited number of AC coefficients are required and the specific unit may be only partially decoded to generate sufficient AC coefficients for retaining acceptable fidelity.
Thus, a need exists for an improved decoding method and apparatus capable of instantaneously bypassing unnecessary AC coefficients for a block during the Huffman decoding process, thereby improving the overall decoding efficiency.